Apple trees are large if grown from seed. Generally apple cultivars are propagated by grafting onto rootstocks, which control the size of the resulting tree. There are more than 7,500 known cultivars of apples, resulting in a range of desired characteristics. Different cultivars are bred for various tastes and uses, including cooking, eating raw and cider production. Trees and fruit are prone to a number of fungal, bacterial and pest problems, which can be controlled by a number of organic and non-organic means. In 2010, the fruit's genome was sequenced as part of research on disease control and selective breeding in apple production.

Worldwide production of apples in 2014 was 84.6 million tonnes, with China accounting for 48% of the total.[3]

Contents

Botanical information

Blossoms, fruits, and leaves of the apple tree (Malus pumila)

An apple's side, stem end, and interior

The apple is a deciduous tree, generally standing 6 to 15 ft (1.8 to 4.6 m) tall in cultivation and up to 30 ft (9.1 m) in the wild. When cultivated, the size, shape and branch density are determined by rootstock selection and trimming method. The leaves are alternately arranged dark green-colored simple ovals with serrated margins and slightly downy undersides.[4]

Blossoms are produced in spring simultaneously with the budding of the leaves, and are produced on spurs and some long shoots. The 3 to 4 cm (1.2 to 1.6 in) flowers are white with a pink tinge that gradually fades, five petaled, with an inflorescence consisting of a cyme with 4–6 flowers. The central flower of the inflorescence is called the "king bloom"; it opens first, and can develop a larger fruit.[4][5]

The fruit matures in late summer or autumn, and cultivars exist with a wide range of sizes. Commercial growers aim to produce an apple that is 23⁄4 to 31⁄4 in (7.0 to 8.3 cm) in diameter, due to market preference. Some consumers, especially those in Japan, prefer a larger apple, while apples below 21⁄4 in (5.7 cm) are generally used for making juice and have little fresh market value. The skin of ripe apples is generally red, yellow, green, pink, or russetted although many bi- or tri-colored cultivars may be found.[6] The skin may also be wholly or partly russeted i.e. rough and brown. The skin is covered in a protective layer of epicuticular wax.[7] The exocarp (flesh) is generally pale yellowish-white,[6] though pink or yellow exocarps also occur.

Wild ancestors

The original wild ancestor of Malus pumila was Malus sieversii, found growing wild in the mountains of Central Asia in southern Kazakhstan, Kyrgyzstan, Tajikistan, and Xinjiang, China.[4][8] Cultivation of the species, most likely beginning on the forested flanks of the Tian Shan mountains, progressed over a long period of time and permitted secondary introgression of genes from other species into the open-pollinated seeds. Significant exchange with Malus sylvestris, the crabapple, resulted in current populations of apples being more related to crabapples than to the more morphologically similar progenitor Malus sieversii. In strains without recent admixture the contribution of the latter predominates.[9][10][11]

Genome

In 2010, an Italian-led consortium announced they had sequenced the complete genome of the apple in collaboration with horticultural genomicists at Washington State University,[12] using 'Golden Delicious'.[13] It had about 57,000 genes, the highest number of any plant genome studied to date[14] and more genes than the human genome (about 30,000).[15] This new understanding of the apple genome will help scientists in identifying genes and gene variants that contribute to resistance to disease and drought, and other desirable characteristics. Understanding the genes behind these characteristics will allow scientists to perform more knowledgeable selective breeding. The genome sequence also provided proof that Malus sieversii was the wild ancestor of the domestic apple—an issue that had been long-debated in the scientific community.[12]

History

The center of diversity of the genus Malus is in eastern present-day Turkey. The apple tree was perhaps the earliest tree to be cultivated,[16] and its fruits have been improved through selection over thousands of years. Alexander the Great is credited with finding dwarfed apples in Kazakhstan in 328 BCE.[4] Winter apples, picked in late autumn and stored just above freezing, have been an important food in Asia and Europe for millennia.[16]

Of the many Old World plants that the Spanish introduced to Chiloé Archipelago in the 16th century, apple trees became particularly well adapted.[17] Apples were introduced to North America by colonists in the 17th century,[4] and the first apple orchard on the North American continent was planted in Boston by Reverend William Blaxton in 1625.[18] The only apples native to North America are crab apples, which were once called "common apples".[19] Apple cultivars brought as seed from Europe were spread along Native American trade routes, as well as being cultivated on colonial farms. An 1845 United States apples nursery catalogue sold 350 of the "best" cultivars, showing the proliferation of new North American cultivars by the early 19th century.[19] In the 20th century, irrigation projects in Eastern Washington began and allowed the development of the multibillion-dollar fruit industry, of which the apple is the leading product.[4]

Until the 20th century, farmers stored apples in frostproof cellars during the winter for their own use or for sale. Improved transportation of fresh apples by train and road replaced the necessity for storage.[20][21]Controlled atmosphere facilities are used to keep apples fresh year-round. Controlled atmosphere facilities use high humidity, low oxygen, and controlled carbon dioxide levels to maintain fruit freshness. They were first used in the United States in the 1960s.[22]

Davidson notes a connection between apples and the Vanir, a tribe of gods associated with fertility in Norse mythology, citing an instance of eleven "golden apples" being given to woo the beautiful Gerðr by Skírnir, who was acting as messenger for the major Vanir god Freyr in stanzas 19 and 20 of Skírnismál. Davidson also notes a further connection between fertility and apples in Norse mythology in chapter 2 of the Völsunga saga when the major goddess Frigg sends King Rerir an apple after he prays to Odin for a child, Frigg's messenger (in the guise of a crow) drops the apple in his lap as he sits atop a mound.[24] Rerir's wife's consumption of the apple results in a six-year pregnancy and the Caesarean section birth of their son—the hero Völsung.[25]

Further, Davidson points out the "strange" phrase "Apples of Hel" used in an 11th-century poem by the skald Thorbiorn Brúnarson. She states this may imply that the apple was thought of by Brúnarson as the food of the dead. Further, Davidson notes that the potentially Germanic goddess Nehalennia is sometimes depicted with apples and that parallels exist in early Irish stories. Davidson asserts that while cultivation of the apple in Northern Europe extends back to at least the time of the Roman Empire and came to Europe from the Near East, the native varieties of apple trees growing in Northern Europe are small and bitter. Davidson concludes that in the figure of Iðunn "we must have a dim reflection of an old symbol: that of the guardian goddess of the life-giving fruit of the other world."[23]

The Greek goddess of discord, Eris, became disgruntled after she was excluded from the wedding of Peleus and Thetis.[30] In retaliation, she tossed a golden apple inscribed Καλλίστη (Kalliste, sometimes transliterated Kallisti, "For the most beautiful one"), into the wedding party. Three goddesses claimed the apple: Hera, Athena, and Aphrodite. Paris of Troy was appointed to select the recipient. After being bribed by both Hera and Athena, Aphrodite tempted him with the most beautiful woman in the world, Helen of Sparta. He awarded the apple to Aphrodite, thus indirectly causing the Trojan War.[31]

The apple was thus considered, in ancient Greece, to be sacred to Aphrodite, and to throw an apple at someone was to symbolically declare one's love; and similarly, to catch it was to symbolically show one's acceptance of that love. An epigram claiming authorship by Plato states:[32]

I throw the apple at you, and if you are willing to love me, take it and share your girlhood with me; but if your thoughts are what I pray they are not, even then take it, and consider how short-lived is beauty.

Atalanta, also of Greek mythology, raced all her suitors in an attempt to avoid marriage. She outran all but Hippomenes (also known as Melanion, a name possibly derived from melon the Greek word for both "apple" and fruit in general),[28] who defeated her by cunning, not speed. Hippomenes knew that he could not win in a fair race, so he used three golden apples (gifts of Aphrodite, the goddess of love) to distract Atalanta. It took all three apples and all of his speed, but Hippomenes was finally successful, winning the race and Atalanta's hand.[27]

Christian art

Though the forbidden fruit of Eden in the Book of Genesis is not identified, popular Christian tradition has held that it was an apple that Eve coaxed Adam to share with her.[33] The origin of the popular identification with a fruit unknown in the Middle East in biblical times is found in confusion between the Latin words mālum (an apple) and mălum (an evil), each of which is normally written malum.[34] The tree of the forbidden fruit is called "the tree of the knowledge of good and evil" in Genesis 2:17, and the Latin for "good and evil" is bonum et malum.[35]

Renaissance painters may also have been influenced by the story of the golden apples in the Garden of Hesperides. As a result, in the story of Adam and Eve, the apple became a symbol for knowledge, immortality, temptation, the fall of man into sin, and sin itself. The larynx in the human throat has been called the "Adam's apple" because of a notion that it was caused by the forbidden fruit remaining in the throat of Adam.[33] The apple as symbol of sexual seduction has been used to imply human sexuality, possibly in an ironic vein.[33]

Cultivars

There are more than 7,500 known cultivars of apples.[36] Cultivars vary in their yield and the ultimate size of the tree, even when grown on the same rootstock.[37] Different cultivars are available for temperate and subtropical climates. The UK's National Fruit Collection, which is the responsibility of the Department of Environment Food and Rural Affairs, includes a collection of over 2,000 cultivars of apple tree in Kent.[38] The University of Reading, which is responsible for developing the UK national collection database, provides access to search the national collection. The University of Reading's work is part of the European Cooperative Programme for Plant Genetic Resources of which there are 38 countries participating in the Malus/Pyrus work group.[39]

The UK's national fruit collection database contains a wealth of information on the characteristics and origin of many apples, including alternative names for what is essentially the same "genetic" apple cultivar. Most of these cultivars are bred for eating fresh (dessert apples), though some are cultivated specifically for cooking (cooking apples) or producing cider. Cider apples are typically too tart and astringent to eat fresh, but they give the beverage a rich flavor that dessert apples cannot.[40]

Commercially popular apple cultivars are soft but crisp. Other desired qualities in modern commercial apple breeding are a colorful skin, absence of russeting, ease of shipping, lengthy storage ability, high yields, disease resistance, common apple shape, and developed flavor.[37] Modern apples are generally sweeter than older cultivars, as popular tastes in apples have varied over time. Most North Americans and Europeans favor sweet, subacid apples, but tart apples have a strong minority following.[41] Extremely sweet apples with barely any acid flavor are popular in Asia[41] and especially Indian Subcontinent .[40]

Old cultivars are often oddly shaped, russeted, and have a variety of textures and colors. Some find them to have a better flavor than modern cultivars,[42] but they may have other problems which make them commercially unviable—low yield, disease susceptibility, poor tolerance for storage or transport, or just being the "wrong" size. A few old cultivars are still produced on a large scale, but many have been preserved by home gardeners and farmers that sell directly to local markets. Many unusual and locally important cultivars with their own unique taste and appearance exist; apple conservation campaigns have sprung up around the world to preserve such local cultivars from extinction. In the United Kingdom, old cultivars such as 'Cox's Orange Pippin' and 'Egremont Russet' are still commercially important even though by modern standards they are low yielding and susceptible to disease.[4]

Cultivation

Breeding

In the wild, apples grow readily from seeds. However, like most perennial fruits, apples are ordinarily propagated asexually by grafting. This is because seedling apples are an example of "extreme heterozygotes", in that rather than inheriting DNA from their parents to create a new apple with those characteristics, they are instead significantly different from their parents.[43]Triploid cultivars have an additional reproductive barrier in that 3 sets of chromosomes cannot be divided evenly during meiosis, yielding unequal segregation of the chromosomes (aneuploids). Even in the case when a triploid plant can produce a seed (apples are an example), it occurs infrequently, and seedlings rarely survive.[44]

Because apples do not breed true when planted as seeds, grafting is generally used to produce new apple trees. The rootstock used for the bottom of the graft can be selected to produce trees of a large variety of sizes, as well as changing the winter hardiness, insect and disease resistance, and soil preference of the resulting tree. Dwarf rootstocks can be used to produce very small trees (less than 3.0 m (10 ft) high at maturity), which bear fruit earlier in their life cycle than full size trees.[45] Dwarf rootstocks for apple trees can be traced as far back as 300 BC, to the area of Persia and Asia Minor. Alexander the Great sent samples of dwarf apple trees to Aristotle's Lyceum. Dwarf rootstocks became common by the 15th century, and later went through several cycles of popularity and decline throughout the world.[46] The majority of the rootstocks used today to control size in apples were developed in England in the early 1900s. The East Malling Research Station conducted extensive research into rootstocks, and today their rootstocks are given an "M" prefix to designate their origin. Rootstocks marked with an "MM" prefix are Malling-series cultivars later crossed with trees of 'Northern Spy' in Merton, England.[47]

Most new apple cultivars originate as seedlings, which either arise by chance or are bred by deliberately crossing cultivars with promising characteristics.[48] The words "seedling", "pippin", and "kernel" in the name of an apple cultivar suggest that it originated as a seedling. Apples can also form bud sports (mutations on a single branch). Some bud sports turn out to be improved strains of the parent cultivar. Some differ sufficiently from the parent tree to be considered new cultivars.[49]

Since the 1930s, the Excelsior Experiment Station at the University of Minnesota has introduced a steady progression of important apples that are widely grown, both commercially and by local orchardists, throughout Minnesota and Wisconsin. Its most important contributions have included 'Haralson' (which is the most widely cultivated apple in Minnesota), 'Wealthy', 'Honeygold', and 'Honeycrisp'.

Apples have been acclimatized in Ecuador at very high altitudes, where they can often, with the needed factors, provide crops twice per year because of constant temperate conditions year-round.[50]

Pollination

Apples are self-incompatible; they must cross-pollinate to develop fruit. During the flowering each season, apple growers often utilize pollinators to carry pollen. Honey bees are most commonly used. Orchard mason bees are also used as supplemental pollinators in commercial orchards. Bumblebeequeens are sometimes present in orchards, but not usually in enough quantity to be significant pollinators.[49][51]

There are four to seven pollination groups in apples, depending on climate:

Group A – Early flowering, 1 to 3 May in England ('Gravenstein', 'Red Astrachan')

Maturation and harvest

Cultivars vary in their yield and the ultimate size of the tree, even when grown on the same rootstock. Some cultivars, if left unpruned, will grow very large, which allows them to bear much more fruit, but makes harvesting very difficult. Depending on the tree density (number of trees planted per unit surface area), mature trees typically bear 40–200 kg (90–440 lb) of apples each year, though productivity can be close to zero in poor years. Apples are harvested using three-point ladders that are designed to fit amongst the branches. Trees grafted on dwarfing rootstocks will bear about 10–80 kg (20–180 lb) of fruit per year.[49]

Farms with apple orchards may open them to the public, so consumers may themselves pick the apples they will purchase.[53]

Crops ripen at different times of the year according to the cultivar. Cultivar that yield their crop in the summer include 'Gala', 'Golden Supreme', 'McIntosh', 'Transparent', 'Primate', 'Sweet Bough', and 'Duchess'; fall producers include 'Fuji', 'Jonagold', 'Golden Delicious', 'Red Delicious', 'Chenango', 'Gravenstein', 'Wealthy', 'McIntosh', 'Snow', and 'Blenheim'; winter producers include 'Winesap', 'Granny Smith', 'King', 'Wagener', 'Swayzie', 'Greening', and 'Tolman Sweet'.[19]

Storage

Commercially, apples can be stored for some months in controlled atmosphere chambers to delay ethylene-induced ripening. Apples are commonly stored in chambers with higher concentrations of carbon dioxide and high air filtration. This prevents ethylene concentrations from rising to higher amounts and preventing ripening from occurring too quickly. Ripening continues when the fruit is removed from storage.[citation needed]

For home storage, most cultivars of apple can be held for approximately two weeks when kept at the coolest part of the refrigerator (i.e. below 5 °C). Some can be stored up to a year without significant degradation.[dubious– discuss][54][verification needed] Some varieties of apples (e.g. 'Granny Smith' and 'Fuji') have more than three times the storage life of others.[55]

Non-organic apples may be sprayed with 1-methylcyclopropene blocking the apples' ethylene receptors, temporarily preventing them from ripening.[56]

Pests and diseases

Apple trees are susceptible to a number of fungal and bacterial diseases and insect pests. Many commercial orchards pursue a program of chemical sprays to maintain high fruit quality, tree health, and high yields. A trend in orchard management is the use of organic methods.[citation needed] These prohibit the use of synthetic pesticides, though some older pesticides are allowed. Organic methods include, for instance, introducing its natural predator to reduce the population of a particular pest.

A wide range of pests and diseases can affect the plant; three of the more common diseases/pests are mildew, aphids and apple scab.

Mildew: which is characterized by light grey powdery patches appearing on the leaves, shoots and flowers, normally in spring. The flowers will turn a creamy yellow color and will not develop correctly. This can be treated in a manner not dissimilar from treating Botrytis; eliminating the conditions which caused the disease in the first place and burning the infected plants are among the recommended actions to take.[57]

Aphids: There are five species of aphids commonly found on apples: apple grain aphid, rosy apple aphid, apple aphid, spirea aphid and the woolly apple aphid. The aphid species can be identified by their color, the time of year when they are present and by differences in the cornicles, which are small paired projections from the rear of aphids.[57] Aphids feed on foliage using needle-like mouth parts to suck out plant juices. When present in high numbers, certain species reduce tree growth and vigor.[58]

Apple scab: Apple scab causes leaves to develop olive-brown spots with a velvety texture that later turn brown and become cork-like in texture. The disease also affects the fruit, which also develops similar brown spots with velvety or cork-like textures. Apple scab is spread through fungus growing in old apple leaves on the ground and spreads during warm spring weather to infect the new year's growth.[59]

World production of apples in 2014 was 84.6 million tonnes, with China producing 48% of the world total (table).[3] Other major producers with 6% or less of the world total were the United States, Poland, Turkey, and Italy.

Human consumption

The whole fruit including the skin, except for the seeds, is suitable for human consumption. The core, containing the seeds, is usually not eaten and is discarded.

Apples can be consumed various ways: juice, raw in salads, baked in pies, cooked into sauces, and other baked dishes.[62]

Several techniques are used to preserve apples and apple products. Apples can be canned, dried or frozen.[62] Canned or frozen apples are eventually baked into pies or other cooked dishes. Apple juice or cider is also bottled. Apple juice is often concentrated and frozen.

Popular uses

Apples are often eaten raw. Cultivars bred for raw consumption are termed dessert or table apples.

In the UK, a toffee apple is a traditional confection made by coating an apple in hot toffee and allowing it to cool. Similar treats in the U.S. are candy apples (coated in a hard shell of crystallized sugar syrup), and caramel apples (coated with cooled caramel).

Apples are eaten with honey at the Jewish New Year of Rosh Hashanah to symbolize a sweet new year.[53]

Apples are an important ingredient in many desserts, such as apple pie, apple crumble, apple crisp and apple cake. When cooked, some apple cultivars easily form a puree known as apple sauce. Apples are also made into apple butter and apple jelly. They are often baked or stewed and are also (cooked) in some meat dishes. Dried apples can be eaten or reconstituted (soaked in water, alcohol or some other liquid).

Apples are milled or pressed to produce apple juice, which may be drunk unfiltered (called apple cider in North America), or filtered. Filtered juice is often concentrated and frozen to be reconstituted later and consumed. Apple juice can be fermented to make cider (called hard cider in North America), ciderkin, and vinegar. Through distillation, various alcoholic beverages can be produced, such as applejack, Calvados, and apfelwein.[63]

Sliced apples turn brown with exposure to air due to the conversion of natural phenolic substances into melanin upon exposure to oxygen.[64] Different cultivars vary in their propensity to brown after slicing[65] and the genetically engineeredArctic Apples do not brown. Sliced fruit can be treated with acidulated water to prevent this effect.[64] Sliced apple consumption tripled in the US from 2004 to 2014 to 500 million apples annually due to its convenience.[66]

Organic production

Organic apples are commonly produced in the United States.[67] Due to infestations by key insects and diseases, organic production is difficult in Europe.[68] The use of pesticides containing chemicals, such as sulfur, copper, microorganisms, viruses, clay powders, or plant extracts (pyrethrum, neem) has been approved by the EU Organic Standing Committee to improve organic yield and quality.[68] A light coating of kaolin, which forms a physical barrier to some pests, also may help prevent apple sun scalding.[49]

Phenolic compounds, such as polyphenol oxidase, are the main driving force behind browning in apples. Polyphenol oxidase catalyzes the reaction of phenolic compounds to o-quinones causing the pigment to turn darker and therefore brown.[70]

Ideain (cyanidin 3-O-galactoside) is an anthocyanin, a type of pigment, which is found in some red apple cultivars.[71]

Phlorizin is a flavonoid that is found in apple trees, particularly in the leaves, and in only small amounts if at all in other plants, even other species of the Malus genus or related plants such as pear trees.[72]

Other products

Health effects

Preliminary research is investigating whether nutrients and/or phytochemicals in apples may be preventive against the risk of some types of cancer.[69][74]

Allergy

One form of apple allergy, often found in northern Europe, is called birch-apple syndrome, and is found in people who are also allergic to birchpollen.[75] Allergic reactions are triggered by a protein in apples that is similar to birch pollen, and people affected by this protein can also develop allergies to other fruits, nuts, and vegetables. Reactions, which entail oral allergy syndrome (OAS), generally involve itching and inflammation of the mouth and throat,[75] but in rare cases can also include life-threatening anaphylaxis.[76] This reaction only occurs when raw fruit is consumed—the allergen is neutralized in the cooking process. The variety of apple, maturity and storage conditions can change the amount of allergen present in individual fruits. Long storage times can increase the amount of proteins that cause birch-apple syndrome.[75]

In other areas, such as the Mediterranean, some individuals have adverse reactions to apples because of their similarity to peaches.[75] This form of apple allergy also includes OAS, but often has more severe symptoms, such as vomiting, abdominal pain and urticaria, and can be life-threatening. Individuals with this form of allergy can also develop reactions to other fruits and nuts. Cooking does not break down the protein causing this particular reaction, so affected individuals can eat neither raw nor cooked apples. Freshly harvested, over-ripe fruits tend to have the highest levels of the protein that causes this reaction.[75]

Breeding efforts have yet to produce a hypoallergenic fruit suitable for either of the two forms of apple allergy.[75]

Proverbs

The proverb "An apple a day keeps the doctor away", addressing the health effects of the fruit, dates from 19th century Wales, according to Caroline Taggart, author of "An Apple a Day: Old-Fashioned Proverbs and Why They Still Work". The original phrase, Taggart said, was: "Eat an apple on going to bed, and you'll keep the doctor from earning his bread". In the 19th century and early 20th, the phrase evolved to "an apple a day, no doctor to pay" and "an apple a days sends the doctor away", while the phrasing now commonly used was first recorded in 1922.[79]

^S. Sansavini (1 July 1986). "The chilling requirement in apple and its role in regulating Time of flowering in spring in cold-Winter Climate". Symposium on Growth Regulators in Fruit Production (International ed.). Acta Horticulturae. p. 179. ISBN978-90-6605-182-9.

^ ab"Apples". Washington State Apple Advertising Commission. Archived from the original on 20 December 2007. Retrieved 22 January 2008.

^ abLowther, Granville; William Worthington. The Encyclopedia of Practical Horticulture: A Reference System of Commercial Horticulture, Covering the Practical and Scientific Phases of Horticulture, with Special Reference to Fruits and Vegetables. The Encyclopedia of horticulture corporation.